A GOOD night's sleep may help to protect against cancer, it was claimed last night.

But restless nights may disrupt hormone balances and make a person more vulnerable to the disease, scientists suggest.

Previous studies have shown that cancer patients who go through group therapy or have a strong social network fare better than those who do not.

According to the new theory, the reason could be that people with stronger social support sleep more soundly.

A team of American scientists led by Professor David Spiegel, from Stanford University in California, argues that sleep can seriously alter the body's hormonal balance.

This made the sleep/wake cycle - also called the circadian rhythm - a good candidate for linking social support with cancer outcomes.

"Psychosocial factors affect your behaviour patterns, such as exercise, what you eat and drink, and your sleep," said Prof Spiegel at the annual meeting of the American Association for the Advancement of Science, in Seattle.

He suggested two ways in which the circadian rhythm might influence cancer progression. The first involved mela- tonin, a hormone produced by the brain during sleep.

Melatonin was an anti-oxidant which mopped up damaging groups of atoms called free radicals. When sleep was disrupted the body produced less melatonin and cells could become more prone to cancer-causing DNA mutations.

Melatonin also slowed production of the female hormone oestrogen, which can help fuel many breast and ovarian tumours. The second link lay with the hormone cortisol, which normally reaches peak levels at dawn and then declined during the day.

A spokesman for Stanford University said, "In past work, Spiegel and his colleagues have found that women with breast cancer whose normal cortisol cycle is disrupted."